1. Field of the Invention
The present invention relates generally to aircraft performance management systems (PMS) for controlling an aircraft in the most efficient manner possible, particularly with the objective of fuel conservation. More specifically, the invention relates to computer apparatus for continuously and accurately providing a measure of the actual enroute weight of the aircraft.
2. Description of the Prior Art
Aircraft weight is one of the most important and critical parameters used in a PMS and the accuracy of such measure contributes significantly to achieving the system's ultimate goal. In the present inventor's copending application entitled "Takeoff Weight Computer Apparatus for Aircraft", Ser. No. 333,098, filed Dec. 21, 1981 and assigned to the present assignee, apparatus is disclosed for computing the takeoff weight of an aircraft. The present invention may use this apparatus for providing a measure of aircraft weight during the ensuing enroute portion of a flight. However, in its broadest aspect, any measure of aircraft takeoff or initial weight may be used, such as for example, its manifest weight, without seriously affecting the accuracy of the enroute weight measure. Also, other techniques for providing a continuous measure of enroute weight have been proposed in the past. Two such techniques are disclosed in the present inventor's U.S. Pat. Nos. 3,691,356 and 4,110,605, both assigned to the present assignee. In both of these patents aircraft weight is computed solely on the basis of the aircraft design parameters and the aerodynamic forces acting on the aircraft and maintaining it in flight; broadly speaking, determining the lift required to support its weight. While these techniques are valid and have proved successful, they are quite complex and require precise measurements of aerodynamic forces and considerable complex computations.
The present invention provides a very accurate measure of enroute weight without the complexity of the prior art systems. In a modern jet transport aircraft in enroute flight, the primary variable affecting aircraft weight is the weight of fuel being consumed. The weight of fuel may, of course, be determined from sensors of the quantity of fuel in the aircraft fuel cells or tanks, which sensors are sufficiently accurate in level, unaccelerated flight to provide an accurate measure of aircraft weight attributable to fuel load. However, since such sensors normally measure the surface level of the fuel in each tank, the measure of fuel will be inaccurate whenever the aircraft is not level (or substantially level) or experiences an acceleration because of fuel surface level changes. Since in enroute flight, extended periods of unlevel and/or accelerated flight may occur, another source of fuel quantity change is required during these periods.